Api577 chapter 10a by Charlie Chong

10 Metallurgy

10 Metallurgy 10.1 GENERAL Metallurgy is a complex science but a general understanding of the major principles is important to the inspector, due to the wide variety of base metals that may be joined by welding during the repair of equipment, and the significant impact on the metals resulting from the welding process. The welding process can affect both the mechanical properties and the corrosion resistance properties of the weldment. This section is designed to provide an awareness of metallurgical effects important to personnel performing inspections, but is not to be considered an in depth resource of metallurgy. Based on the concept that this section provides a basic understanding, this section does not describe all aspects of metallurgy such as crystalline structures of materials and atomic configurations, which are left to other more complete metallurgy texts.

10.2 THE STRUCTURE OF METALS AND ALLOYS Solid metals are crystalline in nature and all have a structure in which the atoms of each crystal are arranged in a specific geometric pattern. The physical properties of metallic materials including strength, ductility and toughness can be attributed to the chemical make-up and orderly arrangement of these atoms. Metals in molten or liquid states have no orderly arrangement to the atoms contained in the melt. As the melt cools, a temperature is reached at which clusters of atoms bond with each other and start to solidify developing into solid crystals within the melt. The individual crystals of pure metal are identical except for their orientation and are called grains. As the temperature is reduced further, these crystals change in form eventually touch and where the grains touch an irregular transition layer of atoms is formed, which is called the grain boundary. Eventually the entire melt solidifies, interlocking the grains into a solid metallic structure called a casting.

Knowledge of cast structures is important since the welding process is somewhat akin to making a casting in a foundry. Because of the similarity in the shape of its grains, a weld can be considered a small casting. A solidified weld may have a structure that looks very much like that of a cast piece of equipment. However, the thermal conditions that are experienced during welding produce a cast structure with characteristics unique to welding.

http://www.sv.vt.edu/classes/MSE2094_NoteBook/96ClassProj/analytic/frontan.html

Fe-4Mo Phase Diagram

Phase Diagram

http://www.azom.com/article.aspx?ArticleID=313

Phase Diagram

http://www.slideshare.net/slideshow/embed_code/27014256 http://www.slideshare.net/piyushvermaiitkgp/metallurgy-basics

10.2.1 The Structure of Castings The overall arrangement of the grains, grain boundaries and phases present in the casting is called the microstructure of the metal. Microstructure is a significant area that inspectors should understand, as it is largely responsible for the physical and mechanical properties of the metal. Because castings used in the refinery industry are typically alloyed, they will contain two or more microstructural phases. A phase is any structure that is physically and compositionally distinct. As the chemical composition is altered or temperature changed, new phases may form or existing phases may disappear. Cast structures, depending on their chemical composition can exhibit a wide range of mechanical properties for several reasons. In general, it is desirable to keep the size of grains small, which improves strength and toughness. This can be achieved by maximizing the rate of cooling or minimizing the heat input (in the case of welding). This increases the rate of crystal formation and decreases the time available for crystal growth, which has a net effect of reducing crystalline grain size.

Weld Grains

HAZ- Heat Affected Zone

HAZ Weld region Fusion Boundary

Coarse grain

Fine grain

Inter-critical

Sub-critical

Unaffected

HAZ http://www.oocities.org/ferrit ec_eng/chapter_1.htm

http://encyclopedia2.thefreedictionary.com/Welded+Joint

Figure 5. Diagram of the heat-affected zone: (I) overheated section, (II) grain-refined (normalized) section, (III) partially grain-refined section, (IV) recrystallized section, (V) aging section; (1) weld metal, (2) fusion zone

1mm / 1mm 0.2mm 0.2+1+1= 2.2 mm

API 582/ NOTE HV10 measurement for HAZ requires 0.04 in. (1 mm) minimum spacing between indentations. In some cases, it is acceptable that hardness measurement location is off-the line in order to satisfy the minimum spacing requirements. Figure 1- Location of Vickers Hardness Indentations

The properties of the cast structure can also be impaired by compositional variations in the microstructure called segregation. Because of the solubility of trace and alloying elements, such elements as carbon, sulfur, and phosphorous, can vary in a pure metal, these elements can cause variations in the solidification temperature of different microstructural phases within the melt. As the melt cools, these elements are eventually contained in the micro structural phases that solidify last in spaces between the grains. These grain boundary regions can have a much higher percentage of trace elements that the grains themselves, which may lead to reductions in ductility and strength properties. This effect can be minimized by using high purity melting stocks, by special melting practices (melting under vacuum or inert gas, for example) to minimize contamination and/or subsequent heat treatment to homogenize the structure. In many carbon steels this is achieved using oxygen scavengers such as aluminum and the steels are often described as “killed” or “fully killed” steels. Minimizing trace elements or “inclusions” at this stage is often important as they can provide sites for formation of in-service defects such as hydrogen induced cracking (HIC).

Steel & Alloy- Grains

Steel & Alloy- Microvoid Coalescence

http://www.power-eng.com/articles/print/volume115/issue-3/features/steam-boiler-inspections-usingremote-field-testing.html

Minimizing trace elements or â&#x20AC;&#x153;inclusionsâ&#x20AC;? at this stage is often important as they can provide sites for formation of in-service defects such as hydrogen induced cracking (HIC).

http://sti.epfl.ch/page-86877-en.html

Gases, such as hydrogen, which become entrapped in the melt as it solidifies, can also affect casting integrity. In some cases, these create voids or porosity in the structure, or can lead to cracking. Weldments are particularly prone to cracking because of trapped hydrogen gases. This problem can be avoided by careful cleaning of the weld bevels to remove hydrocarbons and moisture, the use of low-hydrogen electrodes, correct storage or baking of electrodes and use of proper purging techniques with high quality welding gases. For refinery applications, castings are used primarily for components having complex shapes in order to minimize the amount of machining required. These include pump components (casings, impellers, and stuffing boxes) and valve bodies.(č&#x20AC;&#x192;čŻ&#x2022;é˘&#x2DC;?)

Casting

10.2.2 The Structure of Wrought Materials The vast majority of metallic materials used for the fabrication of refinery and chemical plant equipment are used in the wrought form rather than cast. Mechanical working of the cast ingot produces wrought materials by processes such as; â&#x2013; â&#x2013; â&#x2013;

rolling, forging, or extrusion,

which are normally performed at an elevated temperature. These processes result in a microstructure that has a uniform composition, and a smaller, more uniform grain shape.

Discussion: Why Wrought products are superior than Cast product?

Cast VS. Wrought 1. Cast ingot has less casting defects dues to its simplicity. 2. The cast defects if any will be flatten/dispersed during wrought forming. 3. Post wrought working heat treatment will harmonized the product which may includes stress relieving, grain refinements, modifying the physical and mechanical properties. 4. Others?

Wrought materials may consist of one or more microstructural phases that may have different grain structures. Austenitic stainless steels, for example, are composed of microstructural phase call austenite, which has grains of the same crystal structure. Many nickel, aluminum, titanium and copper alloys are also single-phase materials. Single phase materials are often strengthened by the addition of alloying elements that lead to the formation of nonmetallic or intermetallic precipitates. The addition of carbon to austenitic stainless steels, for example, leads to the formation of very small iron and chromium carbide precipitates in the grains and at grain boundaries. The effect of these precipitates is to strengthen the alloy. In general, greater strengthening occurs with the finer distribution of precipitates. This effect is usually dependent on temperature; at elevated temperatures, the precipitates begin to breakdown and the strengthening effect is lost.

Alloys may also consist of more than one microstructural phase and crystal structure. A number of copper alloys including some brasses are composed of two distinct phases. Plain carbon steel is also a two-phase alloy. One phase is a relatively pure form of iron called ferrite. By itself, ferrite is a fairly weak material. With the addition of more than 0.06 percent carbon, a second phase called pearlite is formed which adds strength to steel. Pearlite is a lamellar (i.e. plate-like) mixture of ferrite and Fe3C iron carbide. As a result of fast cooling such as quenching in non-alloyed steels and also with the addition of alloying elements such as chromium to steel, other phases may form. Rather than pearlite, phases such as (1) bainite or (2) martensite may be produced. These phases tend to increase the strength and hardness of the metal with some loss of ductility. The formation of structures such as bainite and martensite may also be the result of rapid or controlled cooling and reheating within certain temperature ranges often termed “quenching” and “tempering.”

Hypoeutectic Steel - Annealed

Steel & AlloyAnnealed

Pearlite

Steel & Alloy- Annealed Pearlite

Hypoeutectic Steel

1055 Steel Annealed 1055, used in swords and machetes often heat-treated to a spring temper to reduce breakage. It has a carbon content of 0.48-0.55% http://www.bladesmithsforum.com/index.php?showtopic=28855

Pearlite Ferrite

1055 Steel Q&T 1055, used in swords and machetes often heat-treated to a spring temper to reduce breakage. It has a carbon content of 0.48-0.55%

4140 Steel http://en.wikipedia.org/wiki/4140_steel

4140 Steel Annealed

Ferrite

Pearlite

4140 Steel Q&T

10.2.3 Welding Metallurgy Welding metallurgy is concerned with;      

melting, solidification, gas-metal reactions, slag-metal reactions, surface phenomena and base metal reactions.

These reactions occur very rapidly during welding due to the rapid changes in temperature caused by the welding process. This is in contrast to metallurgy of castings, which tends to be slower and often more controlled. The parts of a weld comprises three zones, (1) the weld metal, (2)heat-affected metal (zone), and (3) base metal.

The metallurgy of each weld area is related to the base and weld metal compositions, the welding process and procedures used. Most typical weld metals are rapidly solidified and, like the structure of a casting described earlier, usually solidify in the same manner as a casting and have a fine grain dendritic microstructure. The solidified weld metal is a mixture of melted base metal and deposited weld filler metal, if used. In most welds, there will also be segregation of alloy elements. The amount of segregation is determined by the chemical composition of the weld and the base metal. Consequently, the weld will be less homogenous than the base metal, which can affect the mechanical properties of the weld.

SAW Welding

Dendritic - usually solidify in the same manner as a casting and have a fine grain dendritic microstructure

Dendritic- usually solidify in the same manner as a casting and have a fine grain dendritic microstructure

Dendrite- usually solidify in the same manner as a casting and have a fine grain dendritic microstructure

INCONEL alloy 690 is a high-chromium nickel alloy

Weld Macro- CS

Overlay Welding

The heat-affected zone (HAZ) is adjacent to the weld and is that portion of the base metal that has not been melted, but whose mechanical properties or microstructure have been altered by the preheating temperature and the heat of welding. There will typically be a change in grain size or grain structure and hardness in the HAZ of steel. The size or width of the HAZ is dependent on the heat input used during welding. For carbon steels, the HAZ includes those regions heated to greater than 1350째F (700째C). Each weld pass applied will have its own HAZ and the overlapping heat affected zones will extend through the full thickness of the plate or part welded.

Each weld pass applied will have its own HAZ and the overlapping heat affected zones will extend through the full thickness of the plate or part welded. Unaffected HAZ

http://www.twi-global.com/technical-knowledge/published-papers/investigation-

of-weld-repair-without-post-weld-heat-treatment-for-p91

Overlay Welding

Weld Macro- Fillet Welds

The third component in a welded joint is the base metal. Most of the common carbon and low-alloy steels used for tanks and pressure vessels are weldable. The primary factor affecting the weldability of a base metal is its chemical composition. Each type of metal has welding procedural limits within which sound welds with satisfactory properties can be made. â&#x2013;

If these limits are wide, the metal is said to have good weldability.

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Conversely, if the limits are narrow, the metal is said to have poor weldability.

An important aspect of welding metallurgy is the gas metal reaction between the molten weld metal and gases present during welding. Gas metal reactions depend on the presence of oxygen, hydrogen, or nitrogen, individually or combined in the shielding atmosphere. Oxygen can be drawn in from the atmosphere or occur from the dissociation of water vapor, carbon dioxide, or metal oxide. Air is the most common source of nitrogen, but it can also be used a shielding gas for welding of austenitic or duplex stainless steels. There are many sources of hydrogen. In SMAW or SAW, hydrogen may be present as water in the electrode coating or loose flux. Hydrogen can also come from lubricants, water on the workpiece, surface oxides, or humidity or rain.

An important factor in selecting shielding gases is the type or mixture. A reactive gas such as carbon dioxide can break down at arc temperatures into carbon and oxygen. This is not a problem on carbon and low-alloy steels. However, on high-alloy and reactive metals, this can cause an increase in carbon content and the formation of oxides that can lower the corrosion resistant properties of the weld. High-alloy materials welded with gas-shielded processes usually employ inert shielding gases or mixtures with only slight additions of reactive gases to promote arc stability.

10.3 PHYSICAL PROPERTIES The physical properties of base metals, filler metals and alloys being joined can have an influence on the efficiency and applicability of a welding process. The nature and properties of gas shielding provided by the decomposition of fluxing materials or the direct introduction of shielding gases used to protect the weldment from atmospheric contamination can have a pronounced effect on its ability to provide adequate shielding and on the final chemical and mechanical properties of a weldment. The physical properties of a metal or alloy are those, which are relatively insensitive to structure and can be measured without the application of force. Examples of physical properties of a metal are; ■ ■ ■ ■ ■

the melting temperature, the thermal conductivity, the electrical conductivity, the coefficient of thermal expansion, and the density.

10.3.1 Melting Temperature The melting temperature of different metals is important to know because the higher the melting point, the greater the amount of heat that is needed to melt a given volume of metal. This is seldom a problem in arc welding since the arc temperatures far exceed the melting temperatures of carbon and low-alloy steels. The welder simply increases the amperage to get more heat, thus controlling the volume of weld metal melted per unit length of weld at a given, voltage or arc length and travel speed. A pure metal has a definite melting temperature that is just above its solidification temperature. However, complete melting of alloyed materials occurs over a range of temperatures. Alloyed metals start to melt at a temperature, which is just above its solidus temperature, and, because they may contain different metallurgical phases, melting continues as the temperature increases until it reaches its liquidus temperature.

Alloyed metals start to melt at a temperature, which is just above its solidus temperature, and, because they may contain different metallurgical phases, melting continues as the temperature increases until it reaches its liquidus temperature. http://practicalmaintenance.net/?p=1255

Phase Diagram

Solidus/Liquidus

Phase Diagram

http://www.calphad.com/iron-carbon.html

10.3.2 Thermal Conductivity The thermal conductivity of a material is the rate at which heat is transmitted through a material by conduction or thermal transmittance. â&#x2013;

In general, metals with high electrical conductivity also have high thermal conductivity.

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Materials with high thermal conductivity require higher heat inputs to weld than those with lower thermal conductivity and may require a pre-heat.

Steel is a poor conductor of heat as compared with aluminum or copper. As a result it takes less heat to melt steel. Aluminum is a good conductor of heat and has the ability to transfer heat very efficiently. This ability of aluminum to transfer heat so efficiently also makes it more difficult to weld with low temperature heat sources.

10.3.3 Electrical Conductivity The electrical conductivity of a material is a measure of its efficiency in conducting electrical current. Metals are good conductors of electricity. Metals that have high electrical conductivity are more efficient in conducting electrical current than those with a low electrical conductivity. Aluminum and copper have high electrical conductivity as compared to iron and steel. Their electrical resistance is also much lower, and as a result, less heat is generated in the process of carrying an electrical current. This is one of the reasons that copper and aluminum are used in electric wiring and cables. The ability of steel to carry an electrical current is much less efficient and more heat is produced by its high measure of electrical resistance. One can then deduce that steel can be heated with lower heat inputs than that necessary for aluminum or copper because of its lower measure of electrical conductivity and higher electrical resistance.

The thermal conductivity of a material decreases as temperatures increase. The alloying of pure metals also decreases a materials thermal conductivity. Generally, a material that has had substantial alloying elements added would have a lower thermal conductivity and lower heat inputs are required to raise the material to a desired temperature. More alloy â&#x2020;&#x2019; Lower Conductivity therefore less heat input to melt alloy steel

Aluminum Welds

10.3.4 Coefficient of Thermal Expansion As metals are heated there is an increase in volume. This increase is measured in linear dimensions as the temperature is increased. This linear increase with increased temperature, per degree, is expressed as the coefficient of thermal expansion. An example of this would be the increased length of a steel bar that has been heated in its middle with an oxy fuel torch. As the bar is heated, there will be a measurable increase in length that correlates to the temperature and the specified coefficient of thermal expansion for the material at that temperature. This coefficient of thermal expansion may not be constant throughout a given temperature range because of; 1. the phase changes a material experiences at different temperatures and 2. the increases or decreases in volume that accompany these phase changes.

Metals with a high coefficient of thermal expansion are much more susceptible to warping and distortion problems during welding. The increases in length and shrinkage that accompany the heating and cooling during welding should be anticipated, and procedures established which would assure that proper tolerances are used to minimize the effects of thermal conditions. The joining of metals in which their coefficients of thermal expansion differ greatly can also contribute to thermal fatigue conditions, and result in a premature failure of the component. Welding procedures are often employed, which specify special filler metals that minimize the adverse effects caused by inherent differences between the metals being joined. High coefficient of thermal expansion contribute to; â&#x2013; Distortions, â&#x2013; Thermal fatigues

10.3.5 Density The density of a material is defined as its mass per unit volume. Castings, and therefore welds, are usually less dense than the wrought material of similar composition. Castings and welds contain porosity and inclusions that produce a metal of lower density. This is an important factor employed during RT of welded joints. The density of a metal is often important to a designer, but more important to the welder is the density of shielding gases. A gas with a higher density is more efficient as a shielding gas than one of a lower density as it protects the weld environment longer before dispersion.

Weld porosity

Casting Defects

A gas with a higher density is more efficient as a shielding gas than one of a lower density as it protects the weld environment longer before dispersion.

10.4 MECHANICAL PROPERTIES The mechanical properties of base metals, filler metals and of completed welds are of major importance in the consideration of the design and integrity of welded structures and components. Engineers select materials of construction that provide adequate strength at operating temperatures and pressures. For the inspector, verification that mechanical properties meet the design requirements is essential. Inspectors should understand the underlying principles of mechanical properties and the nature of tests conducted to verify the value of those properties. This is one of the fundamental principles of performing welding procedure qualification tests. Examples of mechanical properties of metals and alloys are, 1. the tensile strength, 2. yield strength, 3. ductility, 4. hardness, and 5. toughness.

Tensile testing â&#x20AC;&#x201C; Tensile /Yield Strength

Charpy impact testing

Charpy Testing Samples

Charpy impact testing

Hardness testing- Sample polishing

Hardness testingHardness tester

Hardness testing

Ductility

Ductility- Bend Tests

10.4.1 Tensile and Yield Strength Tensile testing is used to determine a metals ultimate tensile strength, yield strength, elongation and reduction in area. A tensile test is performed by pulling a test specimen to failure with increasing load. Stress is defined as the force acting in a given region of the metal when an external load is applied. The nominal stress of a metal is equal to the tensile strength. The ultimate tensile strength of a metal is determined by dividing the external load applied by the cross sectional area of the tensile specimen. Strain is defined as the amount of deformation, change in shape, a specimen has experienced when stressed. Strain is expressed as the length of elongation divided by the original length of the specimen prior to being stressed.

When the specimen is subjected to small stresses, the strain is directly proportional to stress. This continues until the yield point of the material is reached. If the stress were removed prior to reaching the yield point of the metal, the specimen would return to its original length and is, considered elastic deformation. However, stress applied above the yield point will produce a permanent increase in specimen length and the yielding is considered plastic deformation. Continued stress may result in some work hardening with an increase in the specimen strength. Uniform elongation will continue, and the elongation begins to concentrate in one localized region within the gage length, as does the reduction in the diameter of the specimen. The test specimen is said to begin to â&#x20AC;&#x153;neck down.â&#x20AC;? The necking-down continues until the specimen can no longer resist the stress and the specimen separates or fractures. The stress at which this occurs is called the ultimate tensile strength.

For design purposes, the maximum usefulness should be a based on the yield strength of a material, as this is considered the elastic/plastic zone for a material, rather than only on the ultimate tensile strength or fracture strength of a material.

Tensile Testing

Note1

A= proportional limit, B=elastic limit, C=upper yield, D=lower yield, E= UTS Note1: It may be noted that stress-strain diagrams are typically based upon the original cross sectional area and the initial gage length, even though these quantities change continuously during the test. These changes have a negligible effect except during the final stages of the test.

Yield point

Sample break

Stress

Tensile Testing

Ultimate tensile strength

Limit of proportionality

Strain

Tensile Testing

http://practicalmaintenance.net/?p=948

Actual tensile testing plot

Elastic/proportional limit /upper/lower yield????

10.4.2 Ductility In tensile testing, ductility is defined as the ability of a material to deform plastically without fracturing, measured by; (1) Elongation or (2) reduction of area.

Elongation is the increase in gage length, measured after fracture of the specimen within the gage length, usually expressed as a percentage of the original gage length. A materials ductility, when subjected to increasing tensile loads, can be helpful to the designer for determining the extent to which a metal can be deformed without fracture in metal working operations such as rolling and extrusion.

The tensile specimen is punch marked in the central section of the specimen, and measured, and the diameter of the reduced area prior to subjecting it to the tensile load is measured. After the specimen has been fractured, the two halves of the fractured tensile specimen are fitted back together as closely as possible, and the distance between the punch marks is again measured. The increase in the after-fracture gage length as compared to the original gage length prior to subjecting the specimen to tensile loads is the elongation of the specimen. This is usually expressed as the percentage of elongation within 2 in. (50.8 mm) of gage length. The diameter at the point of fracture is also measured and the reduction in area from the original area is calculated. This reduction in area is expressed as a percentage. Both the elongation and the reduction in area percentage are measures of a materialâ&#x20AC;&#x2122;s ductility.

The design of items should be based on elastic limit (yield strength). Permanent deformation, resulting from plastic flow, occurs when the elastic limit is exceeded. A material subjected to loads beyond its elastic limit may become strain hardened, or work hardened. This results in a higher effective yield strength, however, the overall ductility based on the strain hardened condition is lower than that of a material which has not been subjected to loads exceeding the elastic limit. Some materials also deteriorate in terms of ductility due to thermal cycling in service. Reduction in ductility in these cases may fall so far that in-service repair welding without cracking becomes very difficult if not impossible. This is sometimes experienced during the repair welding of complex alloy exchanger tubesheets.

One of the most common tests used in the development of welding procedures is the bend test. The bend test is used to evaluate the relative ductility and soundness of welded joint or weld test specimen. The specimen is usually bent in a special guided test jig. The specimens are subjected to strain at the outer fiber by bending the specimen to a specified radius that is based on the type of material and specimen thickness. Codes generally specify a maximum allowable size for cracks in a bend specimen. Cracks and tears resulting from a lack of ductility or discontinuities in the weld metal are evaluated for acceptance or rejection to the applicable code requirements.

Bend Testing

Bend Test Jig

ASTM E290- Guided bend testing

Bend Testing

10.4.3 Hardness The hardness of a material is defined as the resistance to plastic deformation by indentation. Indentation hardness may be measured by various hardness tests, such as Brinell, Rockwell, Knoop and Vickers. Hardness measurements can provide information about the metallurgical changes caused by welding. In alloy steels, a high hardness measurement could indicate the presence of un-tempered martensite in the weld or heataffected zone, while low hardness may indicate an over-tempered condition. There is an approximate interrelationship among the different hardness test results and the tensile strength of some metals. Correlation between hardness values and tensile strength should be used with caution when applied to welded joints or any metal with a heterogeneous structure.

Knoop and Vickers Hardness Tester

Knoop and Vickers Hardness Tester .

http://wes.ir/files/6936303materials%20index.pdf

One Brinell test consists of applying load (force), on a 10 mm diameter hardened steel or tungsten carbide ball to a flat surface of a test specimen by striking the anvil on the Brinell device with a hammer. The impact is transmitted equally to a test bar that is held within the device that has a known Brinell hardness value and through the impression ball to the test specimen surface. The result is an indentation diameter in the test bar and the test specimen surface. The diameters of the resulting impressions are compared and are directly related to the respective hardnessâ&#x20AC;&#x2122;s of the test bar and the test specimen.

Rockwell hardness testing differs from Brinell testing in that the hardness number is based on an inverse relationship to the measurement of the additional depth to which an indenter is forced by a heavy (major) load beyond the depth of a previously applied (minor) load.

The Rockwell test is simple and rapid. The minor load is automatically applied by manually bringing the work piece up against the indenter until the â&#x20AC;&#x153;setâ&#x20AC;? position is established. The zero position is then set on the dial gage of the testing machine. The major load is then applied, and without removing the work piece, the major load is removed, and the Rockwell number then read from the dial. In Rockwell testing, the minor load is always 10 kg, but the major load can be 60, 100 or 150 kg. Indenters can be diamond cone indenters (commonly known as Brales), or hardened steel ball indenters of various diameters. The type of indenters and applied loads depends on the type of material to be tested. A letter has been assigned to each combination of load and indenter. Scale is indicated by a suffix combination of H for Hardness, R for Rockwell and then a letter indicating scale employed. For instance, a value of 55 on the C scale is expressed as 55 HRC.

Vickers hardness testing follows the Brinell principle as far as the hardness is calculated from the ratio of load to area of an indentation as opposed to the depth (the Rockwell principle). Rockwell: Depth of an indentation Vicker: Area of an indentation In the Vickers hardness test, an indenter of a definite shape is pressed into the work material, the load removed, and the diagonals of the resulting indentation measured. The hardness number is calculated by dividing the load by the surface area of the indentation. The indenter for the Vickers test is made of diamond in the form of a square-based pyramid. The depth of indentation is about one-seventh of the diagonal length. The Vickers hardness value is preceded by the designation (HV). The Vickers hardness number is the same as the diamond pyramid hardness number (DPH). In-service hardness testing may involve the use of portable variations of the above-described methods. Alternatively, varying techniques based on rebound, indentation resistance or comparator indentations may be applied and the results related to the hardness scales more commonly accepted. Whatever technique is employed may well be acceptable as long as it produces verifiable and consistent results.

The Vickers hardness value is preceded by the designation (HV). The Vickers hardness number is the same as the diamond pyramid hardness number (DPH). In-service hardness testing may involve the use of portable variations of the above-described methods. Alternatively, varying techniques based on rebound, indentation resistance or comparator indentations may be applied and the results related to the hardness scales more commonly accepted. Whatever technique is employed may well be acceptable as long as it produces verifiable and consistent results. Various codes and standards place hardness requirements on metals and welds. One should compare test results for the material or welding procedures with the applicable standards to assure that the requirements for hardness testing are being met, and that the test results are satisfactory with that specified by the applicable code. There are often in-service degradation requirements, which are hardness related. For example, susceptibility to wet H2S cracking in carbon steel is reduced if hardness levels are maintained below HRC 22.

http://www.twi-global.com/technical-knowledge/job-knowledge/hardness-testing-part-1-074/

10.4.4 Toughness The toughness is the ability of a metal to absorb energy and deform plastically before fracturing. An important material property to tank and pressure vessel designers is the â&#x20AC;&#x153;fracture toughnessâ&#x20AC;? of a metal which is defined as the ability to resist fracture or crack propagation under stress. It is usually measured by the energy absorbed in a notch impact test. There are several types of fracture toughness tests. One of the most common is a notched bar impact test called the Charpy impact test. The Charpy impact test is a pendulum-type single-blow impact test where the specimen is supported at both ends as a simple beam and broken by a falling pendulum. The energy absorbed, as determined by the subsequent rise of the pendulum, is a measure of the impact strength or notch toughness of a material. The tests results are usually recorded in foot-pounds.

The type of notch and the impact test temperature are generally specified and recorded, in addition to specimen size (if they are sub-size specimens, smaller than 10 mm Ă&#x2014; 10 mm). Materials are often tested at various temperatures to determine the ductile to brittle transition temperature. Many codes and standards require impact testing at minimum design metal temperatures based on service or location temperatures to assure that the material has sufficient toughness to resist brittle fracture.

Further Reading

Online Metallurgy Course:

http://www.matter.org.uk/steelmatter/metallurgy/

Online Metallurgy Course

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

http://jpkc.lut.cn/coursefile/cailiaokexuejichu_200 90327/declare.php?action=video http://jpkc.lut.cn/

Online Metallurgy Course

http://210.41.240.168/C1157/sklx4.htm

Online Metallurgy Information

http://www.kobelco-welding.jp/education-center/abc/index.html

http://www.twi-global.com/technicalknowledge/job-knowledge/

Online Metallurgy Course-Steel Making Course - 42 Lectures, 38 Hrs IIT Kanpur

http://www.classiclearn.com/metallurgy/steel-making-course-video_778dfdeef.html

Online Metallurgy Course

http://freevideolectures.com/Course/3104/Principles-of-Physical-Metallurgy/1

Steels - The Structure of Engineering Steels

http://www.azom.com/article.aspx?ArticleID=399

Further Reading: http://practicalmaintenance.net/?p=948

Steels - The Structure of Engineering Steels Topics Covered 1. Introduction 2. Crystal Structures 3. Ferrite 4. Austenite 5. Cementite 6. Pearlite 7. Martensite 8. Bainite

http://www.azom.com/article.aspx?ArticleID=399

Introduction The component elements in steel can be categorized in terms of their crystal structures. At least a basic knowledge of the practical implications of these crystal arrangements is essential to understand the performance of steel in service. The structures are dependent upon the (1) concentrations of each element, (2) the fashion in which the steel is cooled from furnace temperatures, and (3) the amount of cold work performed on the steel. Crystal Structures â&#x2013;

Ferrite

Ferrite (Îą), is the crystal arrangement for pure iron. This form exists as part of the structure in most steels and can usefully absorb carbides of iron and other metals by diffusion in the solid state. Ferrite takes a body centred cubic (bcc) form and is soft and ductile.

Crystal Structures

Phase Diagram

Austenite Austenite (Îł), is a solid solution, that is, the component elements are arranged as if in solution (it also exists as an allotrope of pure iron). All steel exists in this form at sufficiently high temperatures (see figure 1). Some alloy steels stabilise this singular phase and it is present even at room temperatures. The crystal arrangement is face centred cubic (fcc) and, like ferrite, it is soft and ductile.

Cementite Cementite is iron carbide (Fe3C), When carbon atoms can no longer be accommodated in solution in ferrite and austenite (due to an increase in carbon content or reduction in temperature), cementite forms, as it can accommodate more carbon in its crystal structure. Like other carbides, it is hard and brittle.

Austenite

Shows the grain structure of an austenitic stainless steel NF709, observed using light microscopy on a specimen polished and etched electrolytically using 10% oxalic acid solution in water. Many of the grains contain annealing twins. NF709 is a creep-resistant austenitic stainless steel used in the construction of highly sophisticated power generation units. Source: http://www.msm.cam.ac.uk/phase-trans/abstracts/annealing.twin.html

This is a hypereutectoid alloy and illustrates the effect of slow cooling rate on the resultant microstructure. The first phase to form from the melt is proeutectoid cementite. With slow cooling, furnace cooled, allotriomorph cementite forms on the prior austenite grain boundaries, rejecting iron into the melt. At the eutectoid composition the remaining austenite transforms to pearlite.

This secondary electron SEM image shows the cementite delineating prior austenite grain boundaries with a thin layer. The amount of proeutectoid phase is very low, with the majority of the area being taken by the pearlite eutectoid. Again each pearlite cell has a different orientation with the ferrite phase being selectively etched.

Pearlite Pearlite is a phase mixture consisting of alternating platelets of ferrite and cementite (Îą+ Fe3C), which grows by conversion from austenite. A steel containing 0.77 wt% carbon can consist solely of pearlite if cooled sufficiently slowly from austenite (see figure 1). Under the microscope it can have an iridescent mother of pearl appearance, hence the name.

Low carbon steel with a microstructure consisting mostly of ferrite with the darker pearlite regions around the ferrite grains. Upon cooling the steel the ferrite forms initially, either on austenite grain boundaries or inclusions. This causes carbon to be partitioned into the austenite. Eventually the remaining austenite will be at the eutectoid condition and the transformation to pearlite will then take place. This sample has been normalised, removing the directionality caused by casting https://www.flickr.com/search/?q=pearlite

This steel is of the eutectoid composition. Once the temperature is lowered below the eutectoid temperature the steel becomes simultaneously supersaturated with both ferrite and cementite. A eutectoid transformation results (g to a + Fe3C). The resultant microstructure, known as pearlite, comprises lamellae of cementite (dark) embedded in ferrite (white). https://www.flickr.com/search?text=pearlite&sort=relevance

Figure 1. Part of the equilibrium diagram for the Fe-C system A steel containing 0.77 wt% carbon can consist solely of pearlite.

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Martensite Martensite is commonly found in steel that has been rapidly cooled ('quenched') from austenite. It is a particularly hard, brittle arrangement. Essentially it forms because any carbon in solid solution in the austenitic phase at high temperatures does not have enough time to be incorporated into cementite when cooled rapidly. The austenite crystals undergo a transformation involving the shearing of atom planes over each other. Martensite does not appear on the phase diagram (figure 1), as it is not an equilibrium phase.

Martensite https://www.flickr.com/search/?q=martensite

The strain energy involved in the martensitic reaction is enormous and a large undercooling is necessary. In low and medium carbon alloys, the martensite tends to form in lath shaped crystals that are generally too fine to resolve in the light microscope. In high carbon steels, plate martensite forms. For certain steels, the rapid cooling necessary to produce a martensitic structure (e.g. water or brine baths) introduces large surface tensile stresses and may cause quench cracking. However, when medium carbon steels are alloyed with elements such as nickel, chromium and molybdenum, the development of equilibrium phases is suppressed and martensite can be formed with less drastic cooling, such as oil quenching.

Bainite If the steel is cooled such that the formation of pearlite by the short range diffusion of iron atoms is not possible, bainite can be produced. The bainite that forms at temperatures just below those at which pearlite forms is termed upper bainite. At lower temperatures, lower bainite forms. Both lower and upper bainite consist of aggregates of platelets or laths of ferrite, separated by regions of residual phases consisting of untransformed austenite or of phases such as martensite or cementite

Figure 1. Part of the equilibrium diagram for the Fe-C system

FIGURE 2. The TTT diagram for aisi 1080 steel (0.79%c, 0.76%mn) austenitised at 900째c.

Bainite https://www.flickr.com/photos/metalog/5702958915/in/photostream/

Bainite https://www.flickr.com/search?text=bainite&sort=relevance

End of reading!

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Proof Stress Since it is difficult to measure elastic limit, proof stress is the maximum stress a material can withstand without taking more than a small amount of set. The amount is usually specified as the smallest that can be measured by the extensometer, namely, 0.01 percent in 2 inch. As with the yield strength, the preferred method of measuring proof stress is by using a 0.01 percent offset. This is shown in the figure given below (the proportional limit and yield strength are also shown to explain the term clearly).

Re0.2%

Re 0.2% Yield strength is the amount of stress at which plastic deformation becomes noticeable and significant. Fig.1 is an engineering stress-strain diagram in tensile test. Because there is no definite point on the curve where elastic strain ends and plastic strain begins, the yield strength is chosen to be that strength when a definite amount of plastic strain has occurred. For the general engineering structural design, the yield strength is chosen when 0,2 percent plastic strain has taken place. The 0.2% yield strength or the 0.2% offset yield strength is calculated at 0.2% offset from the original crosssectional area of the sample (s=P/A).

STEEL

Steel is an alloy of iron, with carbon, which may contribute up to 2.1% of its weight. Carbon, other elements, and inclusions within iron act as hardening agents that prevent the movement of dislocations that naturally exist in the iron atom crystal lattices. Varying the amount of alloying elements, their form in the steel either as solute elements, or as precipitated phases, retards the movement of those dislocations that make iron so ductile and so weak, and so it controls qualities such as the hardness, ductility, and tensile strength of the resulting steel. Steel can be made stronger than pure iron, but only by trading away ductility, of which iron has an excess.

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